1. Field of the Invention
The present invention relates generally to a digital copying apparatus, and more particularly to a digital copying apparatus of a type including an image reading unit, such as an image scanner, and an image forming unit, such as a laser printer, which is adapted to reduce a required capacity of memory for storing image data and simultaneously to improve the quality of copied images.
2. Discussion of the Background
A digital copying apparatus typically reads a document by an image reading unit such as an image scanner to generate an image signal which is quantized and arranged in a bit map (dot matrix) form in a video memory region in a memory (RAM) as binary data. The binary data is sequentially read from the video memory region and sent to an image forming unit (an engine for driving a laser printer, or the like) as video data for forming an image on recording paper in an electrophotographic method.
In the image signal processing mentioned above, when an image to be formed is represented by analog data, the analog data may be continuous in any direction. However, a digital bit map image, which is produced by quantizing an analog image and arranging quantized binary data in a dot matrix, can only be changed on a dot-by-dot basis stepwisely in orthogonal directions of the dot matrix, so that skewed portions may appear in a resulting image. Particularly, straight lines and smooth curves drawn at any angle to the orthogonal directions of the dot matrix would be produced as jagged lines in a resulting image. This phenomenon is known as "jag" in the art. The jag causes difficulties in accurately forming the same characters and images (particularly outlines of images) as the original image or in forming any characters and images in a desired style.
An effective method for suppressing skewed portions in a bit map image may be implemented by reducing a size of data constituting the dot matrix, to thereby increase the density of the dot matrix and accordingly enhance the resolution of the bit map image. An enhanced resolution, however, results in a significant increase in cost. For example, when a two-dimensional bit map matrix having a size of 300.times.300 dpi (dot per inch) is doubled in resolution, the resulting bit map matrix has a size of 600.times.600 dpi. However, a memory capacity four times larger and a data processing capability, or a data processing speed, four times higher are required for processing data associated with this larger size bit map matrix.
Other methods for suppressing skewed images involve the use of interpolation techniques for interpolating corners of jagged lines to produce a continuous sloping line, or averaging techniques for averaging a brightness of adjacent dots to obscure edges of jagged lines. While these methods can eliminate jagged lines, they also remove fine shapes other than the jagged lines, whereby the contrast and the resolution of resulting images are degraded.
To solve the problems mentioned above, techniques have been developed for selectively adding and removing smaller dots having a dot width smaller than that of a standard dot to and from particular portions of a dot pattern arranged in a bit map matrix to smooth jagged lines. These techniques are described, for example, in U.S. Pat. No. 4,544,922. For realizing such techniques, pattern recognition, template matching, and so on, are utilized as an approach to detect such particular portions subject to the correction in a dot pattern.
While the techniques mentioned above can smooth jagged lines without degrading a contrast of an entire image, to thereby improve image quality, such techniques must perform pattern recognition or template matching on all positions in an arbitrary bit map image and correct each dot in accordance with results of the pattern recognition or the template matching. Thus, a processing apparatus for implementing such techniques is extremely costly and requires a long time for the processing associated with the pattern recognition or the template matching.
To solve the problems mentioned above, Japanese Laid-open Patent Application No. 5-207282 proposes a new image data processing method and apparatus. Specifically, the image data processing method is intended to correct jagged outlines of images in image data arranged in a bit map matrix to improve the image quality as well as to reduce the amount of data to be previously stored in a memory to a minimally required amount. A microprocessor or the like is utilized to determine dots within the image data which requires a correction and to apply correction data to the dots which require the correction. Since the determination can be made by simple processing of the microprocessor, the data correction can be completed in an extremely short time.
More specifically explaining, the image data processing method disclosed in the above patent document 5-207282 recognizes a line shape of a boundary between a black dot region and a white dot region in image data arranged in a bit map matrix, replaces features of the line shape with code information formed of a plurality of bits for each of associated dots, utilizes at least a portion of the code information to determine whether or not each of the dots requires a correction, and applies a correction in accordance with the code information to the dots determined to require the correction.
The code information is representative of the features of the line shape and includes a code indicative of an inclining direction of the line, a code indicative of a degree of the inclination, and a code indicative of a position from a first dot of the line continuous in a horizontal or vertical direction of a dot of interest.
In addition, data on each dot included in a predetermined region centered at the dot of interest in the image data is extracted through a window, and the window is divided into a core region including a central portion of the window and a plurality of peripheral regions around the core region. The code information is generated based on a combination of recognition information derived by image data extracted from the core region and recognition information derived by image data extracted from one or more peripheral regions specified depending on the results of recognition based on the image data extracted from the core region.
An image data processing apparatus for implementing the foregoing image data processing method includes a window for extracting data on each dot in a predetermined region centered at a dot of interest in image data arranged in a bit map matrix; a pattern recognition unit for recognizing the line shape of a boundary between a black dot region and a white dot region in the image data to generate code information formed of a plurality of bits and representing features of the line shape recognized with respect to the dot of interest; a determination unit for determining whether or not a dot requires a correction by utilizing at least a portion of the code information; and a pattern memory for reading and outputting previously stored correction data at an address specified by the code information generated by the pattern recognition unit.
The pattern recognition unit generates, as the code information representative of the features of a line shape, code information including a code indicative of an inclining direction of the line, a code indicative of the degree of the inclination, and a code indicative of a position from a first dot of the line continuous in a horizontal or vertical direction of a dot of interest.
In the image data processing apparatus, the window is divided into a core region including a central portion thereof and a plurality of peripheral regions around the core region. The pattern recognition unit includes a core region recognition unit for recognizing image data extracted from the core region; a peripheral region recognition region for recognizing image data extracted from one or more peripheral regions specified depending on the results of recognition based on the image data extracted from the core region; and a unit for generating the code information based on a combination of recognition information generated by the core region recognition unit and recognition information generated by the peripheral region recognition unit.
According to the image data processing method and apparatus described above, the line shape of a boundary between a black dot region and a white dot region (an outline of a character or the like) in image data arranged in a bit map matrix is recognized, and data on the recognized line shape is replaced with code information formed of a plurality of bits for each of dots included in the boundary. At least a portion of the code information is utilized to determine whether or not each of the dots requires a correction, and a correction in accordance with the code information is made to the dots which require the correction, thereby eliminating the need for previously creating and storing all possible feature patterns requiring a correction as templates. Thus, the determination of dots requiring a correction and the assignment of correction data to the dots requiring a correction can be simply accomplished in a largely reduced time by the use of the code information.
The image data processing method and the image data processing apparatus as described above improve image quality by correcting jagged outlines in image data arranged in a bit map matrix. For this purpose, a required amount of data previously stored in a memory is reduced, and the determination of dots requiring a correction within the image data and the application of correction data to the dots requiring the correction are implemented by simple processing of a CPU or the like, thus making it possible to readily carry out necessary determination and processing associated with the correction of jagged lines in an extremely reduced time.
Japanese Laid-open Patent Application No. 63-56063 proposes a method of determining whether a pixel of interest belongs to a character image region or a halftone image region (identifying a region to which the pixel of interest belongs) based on a ratio of a density level of the pixel of interest to a density level of surrounding pixels.
Further, Japanese Laid-open Patent Application No. 3-70267 describes improvements in gradation characteristic and resolution of a halftone image. Specifically, a binarizing operation generates binary data having a resolution higher than an output resolution, and the binary data is shaped in conformity to the output resolution to improve the gradation characteristic and the resolution of a halftone image.
Furthermore, Japanese Laid-open Patent Application No. 4-156064 describes techniques which determine an edge when a density of a pixel of interest is less than an average density of pixels therearound, and which techniques control error diffusion in a binarizer circuit to prevent a white blank in an image which is more likely to occur in an edge portion having a low density.
A large number of methods have thus been proposed, as mentioned above, for dividing an image into a plurality of regions before the image is binarized for storing associated image data in a memory and for performing processing to enhance the image quality depending on the type of image. However, the division of an image into plural regions before binarization causes an inconvenience that a jagged line to be corrected cannot be distinguished from a halftone region so that a selective correction cannot be made only for a jagged oblique line. Also, while a method has been proposed to change binarizing techniques themselves based on the result of a division of an image, the binarization only is not sufficient to expect an improved image quality. Further, when an engine of a laser printer has a gradation function through modulation of laser diodes, effective utilization of the function is desired.